(1) Field of the Invention
The present invention relates to a cleaning device and an image forming device, and especially to a technology for charging residuals of toner or the like evenly when cleaning an intermediate transfer member.
(2) Description of the Related Art
In the intermediate transfer type image forming devices, toner images are first transferred onto an intermediate transfer member by multiple transfers, and then transferred onto a recording sheet. After the transfer, toner and the like that remain on the surface of the intermediate transfer member are removed by a cleaning device.
As cleaning devices for use for this purpose, various types have been proposed including the following one (for example, see Japanese Patent Application Publication No. 2004-310060). First, an electrically conductive roll brush, while a constant current is applied thereto, is contacted with residual toner on an intermediate transfer belt to cause the residual toner to have a same charging polarity. After this, the residual toner is electrostatically adsorbed to an electrically conductive cleaning brush while a constant current is applied thereto.
The residual toner electrostatically adsorbed to the cleaning brush is then further electrostatically adsorbed to a collection roller, and scraped off therefrom by a scraper.
The toner images having been transferred onto the intermediate transfer belt have the same polarity. As a result, to prevent toner images from repelling each other and prevent a transfer deficiency from being generated due to this, a larger transfer bias is applied as the number of image transfers increases. This makes it unable for the cleaning brush to collect the whole residual toner since the toner remaining on the surface of the intermediate transfer member after the second transfer is greatly varied in the charging state.
In respect of the problem, it has been expected that the amount of uncollected residual toner would be minimized by using a charging brush to charge the residual toner evenly to have the same polarity.
In the actuality, however, it is still observed that a certain amount of residual toner remains uncollected by the cleaning brush. It is considered that this is because the roll brush does not charge the residual toner to a sufficient level of evenness.
A typical roll brush adopted for the above-described purpose is manufactured by wrapping around a core bar a piece of electrically conductive cloth on which a large number of electrically conductive bristles have been planted. However, as shown in FIG. 1, there are gaps 1301 between rolls of the piece of cloth wrapped around the core bar. The gaps 1301 differ from the rolls of the piece of cloth in density of planted bristles.
The gaps 1301 differ from the rolls of the piece of cloth in the efficiency of charging the residual toner. Also, the gaps have a lower charging potential than the rolls of the piece of cloth. It is thus difficult for the cleaning brush to charge the residual toner evenly. The cleaning brush with this structure cannot collect the whole residual toner.
A bar brush can be used instead of a roll brush to charge the residual toner evenly. The bar brush is manufactured by bonding a cloth with bristles with a metal platform by an electrically conductive adhesive. With use of such a bar brush, it is possible to charge the residual toner to a certain level of evenness.
In the case of the bar brush, as shown in FIG. 2, there are gaps between rows of bundles of bristles arranged in the length direction, and there are also gaps between rows of bundles of bristles arranged in the width direction. Gaps 1401 differ from the rows of bundles of bristles in density of planted bristles.
The gaps 1301 differ from the rolls of the piece of cloth in the efficiency of charging the residual toner. Also, the gaps have a lower charging potential than the rolls of the piece of cloth. It is thus difficult for the cleaning brush to charge the residual toner evenly. The cleaning brush with this structure cannot collect the whole residual toner. Also, different from the roll brush, tips of bristles of the bar brush hardly move during the use. As a result, unevenness in the density of planted bristles generates, in a direct relation, unevenness in polarity of the charged residual toner.
When a certain amount of residual toner remains uncollected by the cleaning brush due to the unevenness in polarity of the charged residual toner, an image quality deterioration is generated eventually.
One may consider that the problem might be solved by increasing the number of planted bristles to increase the density of bristles in the gaps of the roll or bar brush. However, this will remarkably increase the density of planted bristles in the portions other than the gaps. The charging brushes with such high density are apt to be clogged with toner. This would create another new problem that the charging efficiency is deteriorated due to the clog, and the life of the charging brush is reduced.